Darwin’s “Abominable Mystery”: Mesozoic Cupules Come to the Rescue?
I have been reviewing recent attempts to solve Darwin’s “abominable mystery”: the abrupt origin of flowering plants (see here, here, and here). Another, even newer study by Shi et al. (2021), published in the prestigious journal Nature, claimed to shed light on the mystery (Chinese Academy of Sciences 2021, Shah 2021). Soltis (2021) commented in the same journal that with this study a part of the puzzle is nearing resolution. This is highly misleading to say the least.
The authors described several hundred extremely well-preserved seed plant fossils from a 125.6-million-year-old Early Cretaceous peat deposit from Inner Mongolia in China. These fossils exhibit recurved cup-like structures called cupules that surrounded the seeds, likely as a protective layer. These cupules have been speculated to be potential precursors of the outer integument as protective layer (Specht & Bartlett 2009), which is unique to the seeds of flowering plants and the reason for their scientific name angiosperms.
Already on the Scene
As mentioned by Soltis (2021), the newly described fossil plants were contemporaneous with true angiosperms already on the scene. Therefore, they could not have been their ancestors but at best a kind of cousins from an ancestral side branch that survived into the era of true flowering plants. Soltis (2021) leaves no doubt about that:
To be clear, the fossils described by Shi and colleagues do not correspond to the immediate ancestors of flowering plants; those fossils remain to be found.
However, according to Shi et al. (2021) these fossils could be related to much older cupule-bearing Mesozoic plants like the seed fern Caytonia, which already existed since in the early Triassic period around 250 million years ago, right after the great end-Permian mass extinction. Caytonia and its relatives (glossopterids and corystosperms) have been suggested in various phylogenetic studies as putative closest fossil relatives of flowering plants (Doyle 2006, Soltis et al. 2008, Rothwell et al. 2009), together forming a clade called angiophytes (rejecting the anthophyte clade with the Bennettitales and the genus Pentoxylon, which are recognized as gymnosperms), which is confirmed by this new study.
In my view this new study therefore may indeed enhance our knowledge about plant relationships and potential homologies of the seed covers. But it most definitely does nothing to solve Darwin’s “abominable mystery” of the abrupt appearance and radiation of flowering plants in the Lower Cretaceous period. But what do I know? So I asked a renowned botanist colleague about this new study and here is what he told me:
“Needless to say, it does not solve the abominable mystery, and the homology between the cupule and outer integument is controversial.” Looks like there is not much to see here in terms of mystery-solving.
Darwin’s “abominable mystery” has significant implications for the controversy pitting Darwinism versus intelligent design theory. Like every theory, Darwin’s theory makes certain predictions, such as the core prediction of gradualism. Gradualism in this context simply means that all big differences in body plans originated through an incremental accumulation of numerous small changes over long periods of time (Richard Dawkins’s metaphor of “climbing mountain improbable”). This core prediction clearly fails the empirical test of the fossil record, which consistently shows discontinuities and abrupt appearances on all levels in all taxonomic groups and all periods of Earth history. Modern paleontological research and statistical tests of the fossil record have shown, beyond a reasonable doubt, that these ubiquitous discontinuities can no longer be explained away as mere artifacts of our incomplete knowledge of an incomplete fossil record. On the other hand, the empirical evidence perfectly agrees with the predictions from intelligent design theory, which postulates pulses of new information infused into the system from outside and thus leading to saltational changes (with or without common descent).
If Charles Darwin, who was much more cautious in his arguments than his modern disciples, were still alive and could see the modern evidence, I think he would likely agree and would no longer despise such conflicting evidence as an abomination. Even more so would the co-discoverer of the theory of natural selection, Alfred Russel Wallace, who was a proponent of intelligently guided evolution anyway.
So, in a nutshell: Darwin’s “abominable mystery” is not only very much alive and kicking, but it also suggests intelligent design rather than an undirected process of Darwinian evolution as the best explanation for the origin of flowering plants and their unique reproductive organs. What’s true for flowering plants is true for all the other groups of life as well, and even more so for the very origin of life itself.
Postscript: There have been some monosulcate angiosperm-like pollen described from Middle Triassic deposits in Norway and Switzerland (Hochuli & Feist-Burkhardt 2004, 2013). However, such monosulcate pollen are also known from the extinct Bennettitales, which according to the more recent studies rather belong to gymnosperms. Therefore, the attribution of these pollen fossils to angiosperms is highly controversial and disputed by some other leading paleobotanists.
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